Single data layer reflective disk technology is well known as audio, computer peripheral, interactive, and video products. For each of these products, information is encoded and recorded as a spiral data track of microscopic pits on the surface of a plastic disk. The pitted surface is covered with a very thin layer of metal to enhance its reflectivity and is then coated with a protective lacquer. These disks are inexpensive to produce using injection molding or embossing methods, and are favored for their large capacity, durability and economy.
Single layer reflective recordings are optically read by reflecting a tightly focused laser beam from the spiral data track using an optical pickup head. The data track is scanned under the head by rotating the disk and the reflected light is measured on a photodetector. The amount of light detected varies as the beam scans over the pits, and the time varying photocurrent signal is amplified and fed to an electronic circuit which decodes the information.
Since any disk warp causes the data track to move out of focus as the disk spins, and since any eccentricity causes the track to wobble radially, a pair of circuits is needed to control the position of the objective lens relative to the data stream, keeping it in focus and centered under the light beam. These circuits are jointly referred to as the "control system" of the optical head and are crucial to its function. A multi-element light detector is usually required to provide information to the control circuit so that it may follow the data stream as it wanders in the two orthogonal directions.
Since the diameter of the focussed spot of the laser beam may not be reduced below a certain diffraction limited minimum, the data density of the optical disk is limited. Data tracks may not be spaced closer together than about the twice the diameter of the focused spot or the signal from a particular track will be contaminated by interference or "crosstalk" from neighboring tracks.
The problem of crosstalk may be exacerbated by various optical aberrations which enlarge the focused spot size beyond its diffraction limited minimum. In particular, spherical aberration, in which marginal light rays are focused at a higher point than central rays due to the influence of the substrate, represents a significant problem. While a known degree of spherical aberration caused by a specified substrate thickness may be corrected by introducing a compensating aberration in the objective lens, any deviation from the specified value will lead to a residual uncorrected aberration. For this reason the thickness of optical disk substrates through which the beam passes must not vary by more than +/-100 micrometers.
A number of previous groups have proposed to increase the data capacity of optical read only disks by stacking multiple data layers on each disk. Multi-layered transmissive disks are described by Bouwhuis in U.S. Pat. Nos. 3,855,426; 3,999,008; 3,999,009; and by Wohlmut et al. in U.S. Pat. Nos. 3,848,095 and 3,946,367. Multi-layered reflective disks are described by Russell in U.S. Pat. Nos. 4,090,031; 4,163,600; 4,219,704; and by Holster et al. in U.S. Pat. Nos. 4,450,553. Multi-layered luminescent disks were also discussed by Russell and Bouwhuis in the above mentioned patents.
The capacity of multi-layered optical storage disks is generally limited by the following considerations:
1. A minimal inter-layer spacing is required to avoid crosstalk between layers. PA1 2. Spherical aberration caused by re-focusing at different depths within the disk in order to access different data layers reduces resolution and requires reduction of transverse data density. PA1 3. The limited working distance of useful objective lenses limits the number of possible layers that may be read without crashing the lens into the disk surface.
As mentioned above, data layers must be separated by sufficient distance that the signal from any particular layer is not contaminated by modulation from its neighboring layers. The rate at which the three dimensional modulation transfer function (MTF) of the optical detection scheme decays in the direction of the optic axis, namely the "degree of optical sectioning", determines the level of crosstalk between data layers as a function of their separation distance. For multi-layered disks described in the above named prior art, these separation distances must exceed 100 micrometers in order to achieve acceptable crosstalk levels since the MTFs fall off rather slowly the detection schemes used.
The relatively large separations between planes in eariler multi-layered disks lead to excessive variation of spherical aberration upon refocusing from layer to layer. A memory with 10 layers, for example, would span about 1 mm thickness, so that upon refocusing from the bottom layer to the top layer substantial spherical aberration would result leading to focus degradation and inter-track crosstalk. While dynamic spherical aberration corrective schemes are possible, such methods may prove difficult and expensive to incorporate in mass produced consumer products. An example of such a corrective scheme involves interposition of a compensating transparent plate of varied thickness between the objective lens and the disk.
In the prior art it has been proposed that multi-layered reflective disks may be read using the same methods as for single layer disks including the "central aperture" detection method and the "push-pull" detection method. In these methods the signal is derived from interference between the zeroth order diffracted beam and higher order diffracted beams.
Wavefront shearing interferometry is a well known optical technique and has been applied as differential interference contrast microscopy for examination of biological, mineral and other specimens. The principles of wavefront shearing interferometry and associated differential interference contrast microscopy are well described in Maksymilian Pluta, Advanced Light Microscopy, Volume 2: Specialized Methods, Chapter 7, pp. 144-197 Elsevier Press, N.Y. (1989). Wavefront shearing interferometry has been applied to reading multi-layered transmissive optical memories as described in Strickler et al., "Three dimensional optical data storage in refractive media by two-photon point excitation", Optics Letters, Vol. 16, No. 22 Nov. 15, 1991, pp. 1780-1782 and also in commonly owned U.S. patent application Ser. No. 07/733,030 filed Feb. 24, 1992.
It is an object of the invention to provide a method and apparatus for reading multi-layered optical information carriers with reduced crosstalk between layers so that the layers may be more closely spaced in order to reduce spherical aberration upon refocusing from layer to layer.
It is a further object to provide reduced crosstalk so as to facilitate tracking and focus control for multi-layered information recovery devices.
It is a further object to allow data layers to be more closely spaced so that more layers may be fit on a single device within the working distance of convenient and inexpensive objective lenses.